It seems that when it comes to fish, the answer is just not clear-cut.

Last week saw the announcement that the world's tiniest fish and vertebrate had been found, measuring a mere 7.9mm.

The little creature, a female of the Paedocypris genus, was discovered in the peat swamps of Sumatra, Indonesia.

But minuscule though this may seem, two other fish reported in the past couple of years claim to be smaller still.

Sexual parasite

In 2004, the Records of the Australian Museum described a male stout infantfish (Schindleria brevipinguis) measuring a diminutive 7mm.

It was found living around Australia's Great Barrier Reef.

At the end of the day, these are all very interesting, very unusual fish that need to be studied in more detail

Dr Ralf Britz, Natural History Museum

And then there was the male anglerfish (Photocorynus spiniceps) discovered in the Philippines. Snout to tail it was just 6.2mm.

The fish was reported in the journal of Ichthyological Research last September.

It spends its life fused to its much larger female counterpart, which in some species are up to half a million times greater in weight.

The stout infantfish was described as about 7mm long

While the female takes care of swimming and eating, the male fish, with its enormous (relatively speaking) testes, is charged with the task of aiding reproduction.

In scientific terms, the male P. spiniceps is labelled a sexual parasite.

Professor Ted Pietsch, from the University of Washington, US, who described the 6.2mm anglerfish, believes this specimen to be the true champion of the small.

Difficult measurements

Dr Ralf Britz, a fish expert at the Natural History Museum in London, UK, was an author on the paper last week that described the 7.9mm Paedocypris vertebrate.

He said the anglerfish was not mentioned in his team's paper because they had simply not seen the P. spiniceps work at the time that it went to press.

The smallest vertebrate [search] has been going on for a really long time

Prof Ted Pietsch, University of Washington

"We just had no idea of this publication; none of us knew about it," Dr Britz explained.

He told the BBC News website that the whole affair raised some interesting questions about how the sizes of such small fish were established.

It was not always clear, he said, when a specimen had reached adulthood and was therefore fully grown.

"It is easier to see how sexually mature a female fish is compared with a male fish. For a male fish, you have to carry out a histological [tissue] section to see if the gonads are ripe," he added.

"Even if you just do a dissection and look at the testes, you can't really tell if they are mature or not, so you need histological studies to prove that."

The Paedocypris fish measures just 7.9mm and is new to science

While the stout infantfish's maturity was not proven using such methods, the anglerfish did have histological evidence that it had reached sexual maturity.

Professor Pietsch, a fish researcher (or ichthyologist), agreed there was some difficulty in describing a creature's size.

"You can use weight, volume or length to describe the smallness of an animal," he told the BBC News website.

"Mine probably isn't the smallest in volume or weight, say compared to the Schindleria [stout infantfish]; but in terms of length it probably is."

Study needed

When asked about the difficulty of measuring a fish that is attached to the back of another, he commented: "You can see exactly where the tip of the snout ends in these tiny males - they are so small they are almost translucent.

"I can't tell you how many times I measured that thing - and if anything 6.2mm is conservative.

"Although this fish lives an unusual existence, it is still a fish; it is still a vertebrate.

"The smallest vertebrate [search] has been going on for a really long time. The three that have been mentioned recently are just the tip of a really long discussion."

And while the quest to find the smallest vertebrate looks certain to continue, Dr Britz is in no doubt about the value to science in investigating such miniature animals.

"At the end of the day, these are all very interesting, very unusual fish that need to be studied in more detail," Dr Britz told the BBC.

"The question over whether they are the smallest or not is not so much a scientific issue but more a popular one."